At this point, we have a working widget. It shows up in the frontend and the right products are displayed for the given category ID.
To configure the category ID, we have to know the ID of the category. We have to copy it from the category page and paste it in the textbox.
For better usability, we will create a custom configuration field to select a category. We will create a button that opens an overlay where we can choose the right category ID.
We will create a similar configuration field that is used for the Catalog Category Link widget type in the backend. You can look at this configuration widget's configuration to see how it works.
Also, ensure that you have the right start files installed ...